1. Field of the Invention
This invention relates to a multilayered container having a multilayered structure, and more specifically, to a multilayered container having excellent delamination resistance, transparency, gas-barrier property an mechanical strength and a process for production thereof.
2. Description of the Prior Art
In recent years, hollow containers of thermoplastic resins have been widely used to hold cosmetics, foods and drinks because of their various advantages such as light weight and safety against bursting. In particular, the development of hollow containers composed of polyethylene terephthalate has rapidly advanced as a result of the improvement of the biaxial stretch-blow molding technique.
Biaxially oriented containers composed of a thermoplastic polyester resin, typically polyethylene terephthalate, do not have all the necessary properties. For example, they have insufficient gas-barrier property to oxygen and carbon dioxide gas, and will impair the flavor of foods and drinks requiring a high level of gas-barrier property.
In an attempt to eliminate this defect, a multilayered-container was proposed which is produced by injecting a thermoplastic polyester and a m-xylylene group-containing polyamide resin (MX nylon) as a gas-barrier thermoplastic resin in this sequence into a single mold from separate injection cylinders to form a three-layered parison composed of an inside and an outside layer of the thermoplastic polyester resin and an inside core layer of MX nylon, and biaxially blow-molding the parison (Japanese Laid-Open Patent Publications Nos. 128516/1982 and 128520/1982; and corresponding U.S. Pat. No. 4,335,901). If in this method, the amount of MX nylon injected is decreased in an attempt to decrease the thickness of the inside core layer, the inside core layer is formed only partly, and the resulting container has insufficient gas-barrier property.
The present inventors developed an improvement over this prior method in which three layers of the thermoplastic polyester resin and two layers of the MX nylon are laminated alternately to form a five-layer structure by injecting the thermoplastic polyester resin, MX nylon and again the thermoplastic polyester resin in this sequence, and consequently, the amount of the MX nylon injected can be decreased from that in the prior art although there are two layers of the MX nylon. This method was applied for a patent (Japanese Laid-Open Patent Publication No. 240409/1985 and corresponding U.S. patent application Ser. No. 731,953 and Japanese Laid-Open Patent Publication No. 108542/1986 and corresponding U.S. Pat. No. 4,728,549).
A method was also proposed in which the thickness of the inside core layer is decreased by first injecting the resin forming the inside and outside layers and then simultaneously injecting the resin forming the inside and outside layers and the resin forming the inside core layer (Japanese Patent Publication No. 16326/1985 corresponding to U.S. Pat. No. 4,174,413). In the resulting three-layer structure, the inside core layer is deviated toward either of the inside and outside layers.
Generally, thermoplastic gas-barrier resin (resin B hereinafter) including MX nylon have poor affinity for resins (resin A hereinafter) such as thermoplastic polyester resins, and the delamination resistance is weak. If a curved surface having a small radius of curvature exists in the rib portion of the container and that curved surface is stretched by the gas pressure of the contents, delamination between resin layers tends to occur at the stretched part. The delaminated part separates into two films, and the container looks slightly white to present an undesirable appearance. The present inventors extensively studied this problem, and finally found that when a curved surface having a radius of curvature of not more than 5 mm on the rib portion of the container is stretched by the gas pressure of the contents, delamination occurs with a high probability in a part composed of five resin layers, whereas it occurs with a very low probability in a part composed of three resin layers; and that the gas-barrier property of the same amount of the gas-barrier resin is better at the part composed of five layers containing two layers of the gas-barrier resin than at the part composed of three resin layers containing one layer of the gas-barrier resin. This finding has now led to the present invention.